Ensure images with no src attribute still returns urls

If a website has images with no "src" attribute, Mechanize
will break when attempting to build relative URLs for images
where image.src returns nil, URI("host") + src will fail

closes sparklemotion#317

lib/mechanize/page/image.rb

@@ -160,9 +160,9 @@ def to_s
   def url
     if relative? then
       if page.bases[0] then
-        page.bases[0].href + src
+        page.bases[0].href + src.to_s
       else
-        page.uri + Mechanize::Util.uri_escape(src)
+        page.uri + Mechanize::Util.uri_escape(src.to_s)
       end
     else
       URI Mechanize::Util.uri_escape(src)

test/test_mechanize_page_image.rb

@@ -180,5 +180,11 @@ def test_image_referer_https_page_rel_src
     assert_equal URI('https://example/'), page.images.first.image_referer.uri
   end
 
+  def test_no_src_attribute
+    page = html_page '<img width="10" height="10" class="foo" />'
+    page.uri = URI 'https://example/'
+    assert_equal URI('https://example/'), page.images.first.url
+  end
+
 end